1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure sensing apparatuses or sensors. More particularly, the invention relates to a direct-type tire inflation pressure sensing apparatus that includes a transmitter with a pressure sensor, which is installed to a wheel of a vehicle and configured to transmit a signal representative of an inflation pressure of a tire on the wheel sensed by the pressure sensor, a receiver installed to the body of the vehicle and configured to receive the signal transmitted from the transmitter, and an acceleration sensor employed to determine a position of the transmitter relative to the receiver so as to improve reliability of the sensing apparatus and decrease power consumption of the same.
2. Description of the Related Art
Conventional tire inflation pressure sensing apparatuses of direct-type generally include a transmitter and a receiver. (For example, Japanese Patent No. 3212311, an English equivalent of which is U.S. Pat. No. 5,602,524, discloses such an apparatus.)
The transmitter, which includes a pressure sensor, is directly installed to a wheel of a vehicle on which a tire is mounted. The transmitter is configured to transmit a signal representative of an inflation pressure of the tire sensed by the pressure sensor.
The receiver, which includes an antenna, is installed to the body of the vehicle. The receiver is configured to receive the signal transmitted from the transmitter through the antenna and determine the inflation pressure of the tire depending on the received signal.
Specifically, with reference to FIG. 11, the transmitter J2 is placed inside the tire J1 and mounted to an air valve J4 of the wheel J3. Consequently, when the vehicle is running, the position of the transmitter J2 relative to the antenna of the receiver varies due to rotation of the transmitter J2 with the tire J1, thus resulting in variation in the level of the signal received by the receiver.
There may exist such a position of the transmitter J2 relative to the antenna of the receiver that the level of the signal received by the receiver is lower than a required level. When the transmitter J2 is rotated to such a position and transmits at the position the signal representative of the inflation pressure of the tire J1, the receiver cannot accurately recognize the signal, thus resulting in a decrease in the fidelity of the receiver.
To avoid such a decrease in the fidelity of the receiver, one may consider applying one of the following two approaches.
The first approach is to determine a suitable installation position of the antenna of the receiver on the body of the vehicle such that the receiver can accurately recognize the signal transmitted by the transmitter J2 at any rotational position of the transmitter J2.
However, determination of such an installation position of the antenna requires an additional task of evaluating the level of the signal received by the receiver.
Moreover, since the signal transmitted from the transmitter J2 will be attenuated during the course of transmission to the receiver, it is, indeed, very difficult to determine such an installation position of the antenna. For example, when the transmitter J2 transmits the signal at a position where the transmitter J2 is opposed to the receiver with the wheel J3 positioned therebetween, the signal cannot pass through the wheel J3 which is made of a metal material. Especially, when the level of the signal transmitted by the transmitter J2 is restricted under regulations, determination of such an installation position of the antenna becomes more difficult.
Furthermore, in the case where the receiver has only a single antenna, through which the receiver receives all the signals transmitted from a plurality of transmitters J2 installed to different wheels J3 of the vehicle, it is required to install the antenna at an optimal position where the receiver can accurately recognize all the signals transmitted from different transmitters J2. However, determination of the optimal position is even more difficult, and a lot of time is required to evaluate the level of all the signals received by the receiver.
The second approach is to allow the transmitter J2 to continuously transmit a plurality of frames, which contain therein the same signal representative of an inflation pressure of the tire, so as to increase the probability of accurate recognition of the signal by the receiver.
However, this approach still cannot certainly ensure the accurate recognition of the signal by the receiver.
Moreover, since a plurality of frames are transmitted for transmission of the same signal, the electrical power consumed by the transmitter J2 is increased. As a result, when the transmitter J2 is supplied with electrical power by a battery, the life of the battery will be shortened.